A ventral view of the abdomen of a 3rd instar nymph of the dragonfly Aeshna palmata. Dragonfly nymphs breath through internal gills called the Branchial Basket by pumping water in and out through the anal opening. The pair of dark wavy lines snaking forward are the main tracheal trunks that carry O2 into the body and CO2 out.

Note that the whole anal area has been colonized by stalked algae cells that are also inside the branchial basket. Might be Oedigonium undulatum which is a known symbiont on Aeshnids.

What is the nature of the symbiosis, that is, what does the dragonfly get out of the deal?

By the way, your description of the setup suggests that this should be posted in macro, not micro. Was the stack shot inside the frame of a microscope, or was the setup macro except for using the objective? I can easily move the thread if appropriate.

By the way, your description of the setup suggests that this should be posted in macro, not micro. Was the stack shot inside the frame of a microscope, or was the setup macro except for using the objective? I can easily move the thread if appropriate.

Rik, you are correct it should be in macro.

I am not positive about the symbiotic relationship, but I suspect that the nymph may get oxygen from the algae. It makes sense to me, considering that they are primarily clustered around the the anal opening, plus internally in the abdomen around the branchial basket. I assume that they can photosynthesize until the nymph develops pigments that block light.

Symbiosis of nymph and green alga — Kammerer [1907] has described an interesting case of symbiosis between the nymph of Aeshna cyanea and the green alga Oedogonium undulatum (Alex. Braun). The nymphs were all found in a small pool near St. Margaretenbad, Bohemia, which had been used for a number of years by the washerwomen of the neighbourhood. All the nymphs of Ae. cyanea in the pool supported a growth of the alga, while none of those found in neighbouring pools not disturbed by the washerwomen were thus affected. A series of experiments showed conclusively that the association was a true case of symbiosis and that the alga-covered nymphs unlike those not associated with the plant were able to live in water strongly contaminated with various impurities, such as soap, their own excreta, etc., by reason of the more abundant supply of oxygen provided by the algae. They were also more resistant towards certain ectoparasites, such as Saprolegnia, which cannot live in an atmosphere rich in oxygen. Thirdly, the growth of algae serves to hide the nymph from its enemies.

On the other hand the alga has the benefit of free transport from place to place, which favours the processes of assimilation, and is able to obtain a richer food-supply from the fecal matter of the nymph and the mud, in which it frequently burrows. The author also suggests that the spines and sharp corners of the chitinous sclerites of the nymph offer favourable points of attachment for the alga and that the latter is protected from many animals which ordinarily feed upon its filaments.

It was also shown by experiments that the association between these two species could be brought about artificially, using individuals originally free-living and independent of one another, but that, with the exception of a temporary attachment to the cyanea nymph of the alga Oedogonium capillare no similar connections could be established between other aquatic larvae and algae. Anax and Libellula nymphs, for example, were refused as hosts by Oedogonium undulatum.

No similar associations have been recorded from among North American Odonata and it is somewhat improbable that they will be found under such circumstances as those attending the case described by Kammerer.

Clearly, the bit about "not recorded in North America" is no longer the case, more than a century later.